Since their debut in the late 1960s and early 1970s, asphalt milling machines have been considered one of the major innovations in road reconstruction. Asphalt milling machines were originally designed to remove a top layer of deteriorated asphalt so a new layer of asphalt could be overlaid on the exposed underlayer. The resulting pavement was superior to simply overlaying a new layer of asphalt directly onto the old and deteriorated asphalt.
One significant benefit of asphalt milling machines that has emerged modernly is the ability to break up asphalt into recyclable-sized fragments. As recycling of all types has become more popular, asphalt milling machines have similarly increased in popularity. In fact, combination milling and paving machines have been developed to mill or break up the old road surface, mix it with new binder, and lay it down to create a new or recycled road surface in one continuous process.
The core component of most modern asphalt milling machines is the cutting drum. Most cutting drums incorporate numerous cutting teeth, coupled to the rounded surface of the drum, to cut or tear into the road surface. The rotational axis of the drum is positioned parallel to the road surface and the drum is rotated while being driven along the road surface in a direction transverse to its axis of rotation. Conventional cutting drums mill the asphalt in an upward direction, or an “up-cut” direction. However, some cutting drums may permit “down-cutting” to control “slabbing,” facilitate pulverizing and mixing, and effectively mill pavement over a wet base. Most cutting drums range in width from 12 to 150 inches and generally have a maximum cutting depth of 4 to 16 inches.
Due to the abrasive nature of pavement, the cutting teeth traditionally wear out quickly and require frequent replacement. The replacement process may create significant downtime and hinder the overall efficiency of the milling process. For example, early cutting drums had cutting teeth that were welded to the drum. Tooth replacement required cutting the old teeth from the drum and welding new teeth in their place. Consequently, considerable effort has been expended to accelerate the replacement process and to increase the durability of the cutting teeth. Many newer cutting teeth, for example, are coupled to the cutting drum using various bolt-on housings to enable faster replacement.
One shortcoming of current asphalt milling machines is their failure to capitalize on cutting-edge technology used in other industries, such as the downhole drilling industry. For example, numerous technological improvements in polycrystalline diamond compact (PDC) bits, which were introduced to the oil and gas industry in the mid 1970s, have enabled PCD bits to capture a growing share of the downhole drill bit market. Some estimates show that between 2000 and 2003, the total footage drilled with PDC bits increased from 26% in 2000 to 50% in 2003. The total revenue generated by PDC bit sales was approximately $600 million in 2003.
Various recent improvements in PDC bit hydraulics, PDC cutter toughness and abrasion-resistance, and PDC bit dynamic stability have resulted in continuous and significant increases in the average rate of penetration (ROP) and bit life of PDC bits, thereby extending the application of PDC bits into harder and more abrasive formations. In some cases, a single PDC bit may drill 20,000 feet or more without replacement. As a result, a PCD bit may save as much as $1 million per well in time-critical drilling applications. It would be a significant advance if drill bit improvements in the downhole drilling industry could be applied to the road reconstruction industry, where downtime and replacement costs incur significant expense.
Accordingly, what are needed are apparatus and methods for incorporating drill bit and other advances of the downhole drilling industry into road reconstruction equipment. More particularly, apparatus and methods are needed to incorporate PCD and other drill bit advances into asphalt milling, grinding, and recycling equipment. Further needed are novel supplemental and auxiliary systems, such as apparatus for depositing new or recycled pavement rejuvenation materials on a roadway, to work in conjunction such apparatus and methods, to facilitate the removal and recycling of asphalt and other pavement materials.